Conventionally, different kinds of communication systems are used to provide voice, data, and video communication services to subscribers. In particular, interactive cable television systems have been developed to distribute information to subscribers' televisions at their requests. One such system concerns a shopping service where subscribers shop at home from an "electronic catalog" or in an "electronic mall". This system is distinguished from home shopping channels since it allows subscribers to select which products or services they will view on their television screens by use of Touch-Tone telephones. Additionally, the subscriber selects the pacing of the product or service display. In operation, the subscriber will tune to a channel and dial a predetermined telephone number to access the system. As soon as the system identifies the subscriber, his television screen begins to display still frame video, and possibly accompanying audio, and directories of "electronic stores" comprising products or services that may be entered or passed, examined in further detail, saved for future consideration, or purchased just by touching keys on a Touch-Tone telephone in response to prompts on the television screen. A mainframe host computer controls the flow of information in the system.
This shopping service uses a CATV network to distribute the video presentations and accompanying audio messages requested by subscribers. At various locations in the CATV network, a device known as a frame store unit captures the video. Each of these locations serves a small group of subscribers. The function of each frame store unit is to capture the video frame destined for one of the subscribers associated with the frame store unit. The frame store unit captures a video frame bearing its address and converts the frame into a form it can store. Next, the frame store unit determines which audio message to associate with the video frame, bundles the audio and video together, and injects the result into the CATV feeder cable. The composite signal transmitted by the frame store unit appears as a standard television picture on the subscriber's television.
Other interactive cable television systems require that each subscriber have his own dedicated channel for viewing video and audio presentations. Still other such systems require the subscriber view such presentations on his own computer terminal.
Disadvantages of these earlier systems include the requirement of a large mainframe computer, and the processing of data in an analog format. The system of the present invention provides the same services as these earlier systems but uses a plurality of powerful, single board computers connected as a local area network ("LAN") in lieu of a centralized computer or mainframe computer.
In a computer network, a large number of separate but inter-connected computers do the work of a single centrally located computer. In any computer network, the inter-connected computers are each autonomous. Several distinct advantages are found in using a LAN over a centralized computer system.
One such advantage concerns the relative price of computing versus communication. Until 1970, computers were relatively expensive compared to communications facilities. The reverse is now true. With the cost of small computers being relatively negligible, it has become attractive to analyze the data at the location where it originates. Analyzing data at the place of origination reduces communication costs, which now represent a larger percentage of total cost than it did in the past.
Another advantage of computer networking is the superior price/performance ratio of small computers over large mainframe computers. Mainframe computers are approximately a factor of 10 times faster than the largest single-chip microprocessor, but they cost a thousand times more than the single chip microprocessor. This imbalance in the price/performance ratio makes it more attractive to use many microcomputers located close together (a LAN) to out-perform the large mainframe computer.
In addition to a favorable price/performance ratio, LANs have other advantages over a single centralized system. For one thing, they are more reliable than a centralized computer system since a single hardware or software failure in a LAN will only bring down one processor, and not affect the others.
Another major advantage of building large computer systems by coupling large numbers of small processors together is the expectation of a simpler software design. In a computer network, it is possible to dedicate some, or all, of the processors to specialized functions. That is, instead of having the processors multi programmed, each processor does only one function at any time. By eliminating the need for multi-programming, software complexities associated with the large mainframe computers are eliminated.
A further advantage of LANs is their ability to increase system performance gradually as the work load increases by merely adding more processors.
Accordingly, the major advantages of using a LAN in the system of the present invention are (1) a more favorable price/performance ratio, (2) acceptable degradation in performance upon failure of a processor, and (3) the practicality of incremental growth.
Also, the system of the present invention receives, processes and transmits data in a digital format as contrasted to the earlier systems which substantially processed data in an analog format. The advantages of a digitized system as compared to an analog system are numerous; for example, (1) equipment and circuits for digital systems have been declining in cost by a factor of two every three years for about the last twenty years, while the cost of analog circuits have not declined as fast, (2) digital signals are easier to multiplex than analog signals, and (3) digital signals, because they are represented by pulses of well defined and uniform shape, are easy to store and regenerate thereby permitting accurate reproduction of the original signals.
Thus, the use of digitally formatted signals and the incorporation of a LAN make the system of the present invention suprior and more cost efficient than the earlier systems.